Axial and Lateral Resolution: Chapter 10

Question 1

what is resolution

Answer 1

the ability to image accurately

Question 2

what is axial resolution?

Answer 2

the ability to distinguish 2 structures that are close to each other front to back, parallel to, or along the beam’s main axis best

Question 3

what units is axial resolution measured in? can it be changed by the sonographer? what is the typical value?

Answer 3

distance = mm or cm; no, a new transducer is needed to change this; 0.05 to 0.5 mm

Question 4

what can provide better axial resolution? why is this?

Answer 4

shorter pulses = short spatial pulse length/ short pulse duration; images are more accurate w shorter pulses. backing material in transducers are designed to have few cycles per pulse so axial resolution is low which = better accuracy

Question 5

what is the equation for axial resolution?

Answer 5

axial resolution (mm) = spatial pulse length (mm)/2

Question 6

how can axial resolution be improved? how can these factors be changed?

Answer 6

1. less ringing, fewer cycles in pulse
2. higher frequency sound, shorter wavelength
   BUT changing either of these requires a new transducer

Question 7

what is lateral resolution?

Answer 7

the minimum distance that 2 structures are separated by side-to-side or perpendicular to the sound beam that produces 2 distinct echoes

Question 8

what unit is lateral resolution measured in?

Answer 8

distance = mm

Question 9

lateral resolution = _______________ what does this mean? what is it called?

Answer 9

beam diameter; since beam diameter caries with depth, the lateral resolution also caries with depth; beam width variation or point spread artifact.

Question 10

where is lateral resolution best? why?

Answer 10

at the focus or one near zone length (focal depth) from the transducer bc the sound beam is narrowest at that point.

Question 11

what happens to lateral resolution as depth changes?

Answer 11

lateral resolution degrades at deeper depths (in the far zone)

Question 12

which is best lateral or axial resolution? why is that?

Answer 12

axial resolution bc sound pulses are wider than they are short.

Question 13

how does high frequency sound affect axial and lateral resolution?

Answer 13

improves axial in entire image; improves lateral in the far, far field (depths greater than 2x the focal depth)

Question 14

what compromises have to be made when dealing with diff frequencies?

Answer 14

higher frequencies improve image detail, low frequencies provide deeper penetration. frequency is chosen based on imaging depth

Question 15

how does focusing alter the US beam?

Answer 15

1. narrower ‘waist”
2. shallower focus
3. smaller focal zone

Question 16

where is focusing effective?

Answer 16

in the near field and the focal zone

Question 17

what are the two types of focusing?

Answer 17

1. fixed- aka conventional or mechanical
2. adjustable- by electronics - aka phased array

Question 18

what are the 3 specific methods of focusing?

Answer 18

FIXED:
1. lens - external focusing
2. curved piezoelectric crystal - internal focusing
ADJUSTABLE:
3. electronic focusing - dynamic, variable focusing or multi-focusing

Question 19

what is true about fixed-focus transducers?

Answer 19

have the poorest lateral resolution bc the focal depth is fixed and cannot be adjusted

Question 20

single crystal transducers are always __________

Answer 20

fixed focus

Question 21

what is true about phased array transducers?

Answer 21

have better overall lateral resolution bc the sonographer can move the focus to the anatomic region of clinical importance.